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    <script>
        // 1. charAt(index) 根据位置返回字符
        var str = 'andy';
        console.log(str.charAt(3));
        // 遍历所有的字符
        for (var i = 0; i < str.length; i++) {
            console.log(str.charAt(i));
        }
        // 2. charCodeAt(index) 返回相应索引号的字符ASCII值 目的： 判断用户按下那个键
        console.log(str.charCodeAt(0)); //97

        // 3. str[index] H5新增 支持IE8+
        console.log(str[0]);
    </script>
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